Beta webapp My Mile Marker (M3) tracks your fuel purchases and odometer readings and calculates your gas mileage. Sign up for a (now) free account at the M3 beta, enter your car(s), and their odometer readings. Each time you visit the pump, enter your current odometer reading, the number of gallons purchased and the price per gallon. You can add your data via the web site or via SMS while you're at the gas station with Twitter.